The present invention relates to warewashing machines, and more particularly, to domestic or household-type dishwashers. Food ware is cleaned in such machines by a sequence of one or more wash and rinse periods under the control of a timer. During a wash period, water and detergent are introduced into the wash chamber of the dishwasher, and this wash fluid is sprayed under pressure onto the food ware by a recirculating pump which pumps the wash fluid through the nozzles of a rotating wash arm system. At the end of each wash period the soiled wash fluid is drained. For rinsing, clean water alone is introduced into the wash chamber, and this rinse fluid is also recirculated and sprayed onto the food ware items, and then drained. Normally, several rinses are required.
Such machines generally have several different operational modes or "cycles", with the number of wash and rinse periods for each being determined by the soil conditions and the quantities or types of articles typically washed in such a cycle. For example, a dishwasher such as shown in U.S. Pat. No. 3,549,294 (assigned to the assignee of the present invention) enables the machine operator to select any of several wash cycles having different time periods and different numbers of wash and rinse periods. In the '294 machine, a Normal Wash is typically used to clean dishes, glasses, and other dinnerware, while a Soak cycle is preferably used for removing heavily baked-on encrustations from pots, pans, or casserole dishes which have been used in cooking or baking. While the total quantity of food soil removed during any particular cycle is related, of course, to the number of food ware items placed within the warewasher and the extent to which the machine operator may already have scraped food soil from the items before placing them in the warewasher, it is normally expected that more soil will be removed during a Soak cycle than during a Normal cycle. As a result, a Soak cycle will typically include a static soak period and more wash and/or rinse periods than a Normal cycle.
The need for several wash and rinse periods results from using the same single wash chamber and recirculating and spraying system for both the washing and rinsing phases of the warewashing operation. No matter how well the fluid may be filtered as it is used, some of the food soil unavoidably becomes suspended within the fluid and then passes continually through the recirculating pump and spraying system as the fluids are being sprayed onto the food ware items. Many of these food soil particles are then redeposited onto the food ware items as they are being washed and/or rinsed. Some also remain behind on the walls of the wash chamber and within the recirculating pump and spray arms. Multiple rinse periods help reduce this redeposit problem, since, during each rinse period, fresh water is introduced, sprayed, and then drained, so that less and less of this fine soil remains, and less and less is redeposited.
In a common method of washing dishes in a domestic dishwashing machine the fluid which is sprayed onto the food ware is first finely filtered of soil to enable the use of small wash arm orifices (typically as small as 4 mm across) and a fairly high pressure pump. Without fine filtering, small orifices and acceptably sized pumps could not be used, and high spray pressures and velocities could not be reliably achieved, due to the likelihood of clogging such small orifices. A small orifice/high pressure system therefore usually requires a fine filter for capturing rather than recirculating the soil. Such a system also typically uses two pumps, one for pumping the filtered fluids through the wash arms, and another, located essentially upstream of the filter, for pumping the water and collected soils to a drain at the end of a wash or rinse period.
However, with systems employing a fine filter the recirculating fluid necessarily passes through food soil which has been captured by the filter. In heavy soil conditions, it is possible for the fluid flow to become sufficiently obstructed at the filter, as by partial clogging thereof, to impair the efficiency and effectiveness of the recirculating and spraying system. It is therefore desirable to remove as much of the soil as possible to a remote location separated from the recirculating and spraying system. This not only prevents clogging of the recirculating and spraying system, but also minimizes disintegration and emulsification of the food soil at the filter, caused by turbulence in the fluids, which continuously agitates the collected food soil debris.
The general principle of separate soil removal is widely recognized in a number of other fields. For example, in many oil-lubricated machines, such as internal combustion engines, in which the lubricating oil is recirculated, a portion thereof is circulated through a bypass filter, thus extending the effective life of the lubricant and of the engine or machine itself. Similarly, in the clothes washing art, an auxiliary water recirculating path is often provided to pass some of the water through a lint filter for subsequent separate removal.
Thus, the more soil removed from the primary recirculating and spray system within the warewasher, and the faster it is removed, the fewer wash and/or rinse cycles--and hence the less hot water--required. Further, by prolonging the usefulness of the water, early soil separation and removal can reduce the amount of hot water needed in each particular period. As a result, energy and water savings can be realized both in the quantity of water consumed during each wash or rinse period, and in the total number of periods required.
The prior art relating to domestic dishwashing machines includes several examples of filters intended to supply the advantages of a bypass filter in accordance with the above discussion. However, they are generally only partially effective, or require servicing by the machine operator, or both. Preferably, such a system should remove large as well as small soil particles from the recirculating and spray system as quickly as possible (without first requiring them to pass through the recirculating spray system itself), and should flush them completely down the drain during draining of the fluid from the warewasher, with essentially no intervention or assistance from the machine operator.